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How much of the heat wave is from climate change?

According to a study published in Nature Climate Change, deadly heat waves are going to be a much bigger problem in the coming decades.

Home plate umpire Bruce Dreckman wipes his face with a towel in the middle of the fourth inning of a baseball game between the St. Louis Cardinals and the San Francisco Giants on Saturday in San Francisco. The Western United States baked through the unofficial end of summer, as temperatures in some parts threatened to peak Saturday at levels not seen in decades.(Photo: Ben Margot, AP)

Is the heat wave scorching Northern California this weekend caused by climate change?

Not exactly, say scientists. Heat waves, droughts and hurricanes like Harvey, which brought historic floods to Texas in recent days, have always been part of the weather in the United States.

But climate change — the steady warming of the Earth from the burning of fossil fuels, which traps heat in the atmosphere — is making them worse. And brutally hot weather like this weekend’s heat wave is almost certain to become more commonplace in the coming decades.

“This is expected behavior. It’s not some vast scientific surprise,” said Ben Santer, a climate scientist in San Ramon and member of the National Academy of Sciences. “We know that by burning fossil fuels we are increasing heat-trapping gases in the atmosphere, and that’s warming the lower atmosphere and Earth’s surface. The expectation has been that we are going to see more heat records, and that’s what we are seeing.”

The number of heat records in recent years is startling.

From 2000 to 2009, there were twice as many record daily high temperatures as daily record low temperatures in the United States, according to an analysis of hundreds of thousands of temperature records at 1,800 weather stations across the country done by the National Center for Atmospheric Research. So far in 2017, there have been nearly four times as many record hot days as record cold days. By 2050, at the current rates of warming, there are expected to be 20 times the number of heat records as cold records, the study found, and 50 times as many by 2100.

“The fact that the Earth is warming is the most solid thing we know about climate change,” said Adam Sobel, a professor of physics at Columbia University and director of its Initiative on Extreme Weather and Climate. “So heat waves have a significant increase in probability. They are being made more probable and more intense by climate change.”

California has warmed about 1.5 degrees Fahrenheit since 1895.

That might not sound like much, but it’s enough to make heat waves a few degrees hotter, which worsens fire risk and smog levels, and puts crops, like wine grapes that can spoil at extreme temperatures, at greater risk.

The Paris Climate Agreement, which President Donald Trump withdrew from earlier this year, set a goal of limiting warming to 2 degrees Celsius — or 3.8 degrees Fahrenheit — by the end of this century. If warming and fossil fuel use continue at current levels, the planet could warm 7 or 8 degrees Fahrenheit by 2100.

Despite the political debate, there is no debate among mainstream scientists.

The 10 hottest years globally back to the 1880s all have occurred since 1998, according to NASA and NOAA. Overall, 2016 was the hottest year ever recorded in California since modern temperature records were first taken in 1895. The previous record for statewide average temperature was 2015, and the record before that was set in 2014.

Hotter temperatures make droughts more severe. They make hurricanes worse, because the ocean, which has risen 8 inches in the past 100 years, surges further inland during storms, and those storms pick up more moisture from evaporation. At the most basic level, they tip very hot weather into hotter weather.

“It’s like you’ve got a ball that’s rolling down a bowling alley, and if you release it closer to the right-hand gutter, you are more likely to get a gutter ball,” said Chris Field, director of Stanford University’s Woods Institute for the Environment.

“We are closer to the gutter now. Sometimes you’ll still get a strike, but we are tilting the tables so that we are more and more likely to get a gutter ball — in other words, more likely to get the extremely hot temperatures and less and less likely to get the cold extremes.”

And at the current rates of warming, things begin to look rather alarming, experts say.

From 1950 to 1970, temperatures in Walnut Creek, for example, topped 100 degrees six days a year, on average. But from 2006 to 2016, there were nine days on average. And unless there are significant reductions in greenhouse gases, from 2030 to 2050 there will be 16 days a year over the century mark, and a sweltering 32 days a year by 2100, according to a database of temperature statistics and carbon dioxide emissions scenariosbuilt by the Scripps Institution of Oceanography and the University of Washington. The numbers are similar for every other major California city.

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Meanwhile, however, California is leading the nation in reducing greenhouse gases, with its emissions down about 8 percent over the past 10 years, even as the economy has grown. Pushed by Gov. Jerry Brown, who has made climate change the centerpiece of his current term in office, the state is on track to hit a target of generating 33 percent of its electricity from solar, wind and other renewable sources by 2020, with that target increasing to 50 percent by 2030. A bill now pending in the Legislature would increase that to 100 percent by 2045.

But even if the rest of the U.S. and the world follows California’s lead, some amount of warming is a certainty in the coming decades. That will mean places like Las Vegas, Phoenix, Sacramento and other cities that are now hitting 110 degrees on some days, will have more.

Before air conditioning was invented, few people moved from the East to places like Las Vegas. Major changes can help reduce the impacts of heat waves, particularly on the elderly and the poor, like planting millions of urban trees, painting rooftops white and coloring pavement lighter colors, said Field. But if it becomes unbearably hot in some places in the decades to come, America may see more migrations.

“People may vote with their feet,” said Field. “Climate has a big influence about where people want to live.”